It usually means that the previews are out of sync with the previews database. IME the only way to cure this is to rebuild all your previews. Delete your previews folder, including the previews.db and rootpixels.db, select All Photographs, Library/Preveiws/Render Previews/Build All previews. Probably best to clear the ACR cache as well. With Standard previews it won't take too long, but with 1:1s be prepared for a long wait. I had the same problem last week, and rebuilding 65K of 1:1 previews took over 2 days (and nights).

In Windows the previews folder (xxxxpreviews.lrdata) contains the actual previews and the previews.db and rootpixels.db. But I see you are on a Mac, and I do seem to remember that on Macs there is just one big previews file? that presumably contains the lot in some form or other. Sorry I can't be more precise, but I've never had a Mac so don't know its peculiarities.